This invention relates to conveyors and has particular application to belt conveyor drives.
In many applications belt conveyors may extend over long distances and may be required to carry very heavy loads. Coal and ore conveyors, for example, may extend over long distances from mining sites to shipping or rail facilities. Such conveyors are generally formed by long flexible belts arranged in endless loops with an upper load carrying runs and lower return runs, the upper runs being supported at intervals by idler roller sets, commonly in a troughed configuration. Each belt loop is driven by a roller drive at the forward end of the loop in the direction of travel of the upper run. With this arrangement tensile forces in the upper loaded run of the belt increase continuously throughout the length of the run to reach a maximum at the front end. This build up of tensile forces puts a practical limit on the length of each belt loop. With modern steel reinforced conveyor belts this practical limit is of the order of 15 kilometres, depending on the terrain.
It has been known to provide intermediate drives for long conveyor belts for the purpose of applying traction forces to loaded belt runs at intervals along their length so as to limit the build up of tensile forces in the belt and permit the use of longer belt runs. These previous proposals have involved use of friction drive belts engaged with the underside of the loaded run of the main conveyor belt. However, such previously proposed intermediate drives have not been satisfactory. In order to generate effective traction forces the friction belt must engage the conveyor belt over quite a long distance. Moreover, discontinuities in the load in the conveyor belt reduce the interface pressure between the conveyor belt and the friction drive belt with consequent loss of traction and rapid build up of excessive tensile forces in the conveyor belt. As a consequence such intermediate drive equipment has not been widely adopted.
There have been previous proposals to provide intermediate drive equipment in which traction elements are held to the conveyor belt by suction clamping. Such equipment is disclosed in U.S. Pat. No. 2,650,695 to SAMUEL D. ROBINS and U.S. Pat. No. 2,692,041 to W. S. CAMPBELL. In the Robins equipment, the traction elements are in the form of individual metal trays carried on drive chains to engage the underside of the conveyor belt and defining suction cups which can be evacuated. The suction cups are formed in two rows extending one along each side of the conveyor belt and are evacuated through individual vacuum tubes permanently connected to the trays and extending to a common vacuum manifold disposed to one side of the conveyor belt. Stationary suction nozzles project through slits in the moving manifold to create a localized vacuum zone within the manifold such that the suction cups are evacuated only when in contact with the conveyor belt.
The Campbell equipment comprises a traction belt provided with lateral grooves which are evacuated by stationary suction nozzles inserted between the edge margins of the conveyor belt and traction belts.
The Robins and Campbell arrangements are quite complicated and involve sliding vacuum seals which would be difficult to maintain under rugged service conditions. Moreover, the application of vacuum from the sides of the conveyor belt severely limits the suction clamping effect which can be achieved and in particular the tractive effort that can be applied to the central region of the conveyor belt where the loading is heaviest.
United States Pat. Nos. 2,314,254 and 2,549,729 to J. G. WALLNY both disclose an apparatus in which an auxiliary belt engages a conveyor belt and has openings to which vacuum is applied by means of a stationary suction hose over which the auxiliary belt slides. However, this apparatus is designed as a belt filter for relatively light duty applications. The conveyor belt is perforated by drainage openings and the auxiliary belt is a relatively thick narrow belt, the vacuum being applied via the suction holes in the auxiliary belt to the drainage holes in the conveyor belt to promote efficient drainage of material carried on the conveyor belt. Moreover, the auxiliary belt is used as a traction belt in order to drive the conveyor belt only in the apparatus disclosed by the later patent issued to Wallny.
By the present invention it is possible to provide a conveyor belt intermediate drive for heavy duty applications in which the tractive forces which can be applied effectively are significantly increased by a suction clamping technique without the need for complicated vacuum connections.